In the conventional art, a toner for use in electrophotography (hereinafter referred to simply as toner) produced such as to eliminate the edge effect, i.e. the density of black solid areas in the images is thinner in the center than in the periphery, and such as to prevent the background of the images from being stained, is disclosed in Japanese Publication for Unexamined Patent Application (refer to for example Publication for Unexamined Patent Application No. 1983-40557, Tokukaisho No. 58-40557, and Publication for Unexamined Patent Application No. 1983-68047, Tokukaisho No. 58-68047). Such a toner is generally composed of toner particles produced by mixing, kneading, grinding and classifying different materials, and of an external additive attached on the surface of the toner particles. This external additive is composed of tin oxide or the like, has a conductivity control function and controls the electrostatic charge of the toner.
The toner particles, when used in a two-component developer composed of toner particles and carrier particles, comprise internal additives such as binder resins for holding the different materials composing the toner together, and for fixing colorants on transfer paper, colorants for imparting color to the toner, electrostatic charge control agents for giving an electrostatic charge to the toner, surface lubricants for preventing the toner to adhere to the heat roller of the electrophotographic apparatus, and other internal additives.
Meanwhile, the toner particles of a single-component developer that does not use carrier particles and where magnetism is given to the toner particles themselves, comprise internal additives such as binder resins, magnetic substances for imparting magnetism to the toner particles, colorants, and other internal additives.
However, when like in the conventional art, tin oxide is added on the surface of the toner particles, the flowability of the toner worsens and a bridge phenomenon (particles of toner are bond to each other) occurs thereby impeding a smooth supply of toner. The toner of the conventional art thus presents the disadvantage that fine images cannot be secured.
Besides, in common copying machines of the electrostatic image transfer type, especially copying machines that employ an organic optical semiconductor as photoreceptor, the surface of the photoreceptor deteriorates due to ozone produced by the electrostatic charger. This causes the toner to form a film on the photoreceptor, and thereby the image to become unclear and fuzzy.